The gas discharge light sources constructed with symmetric or asymmetric arrangement of the gas discharge vessel, i.e. the gas discharge lamps have been known since longer time. Whatever kind of arrangement is applied, the general problem of these light sources is experienced, i.e. the asymmetric length of the current transient after the ignition has been finished. This means the asymmetry of the transition process from the glow discharge state to the normal discharge state. This problem follows mainly from the influence of the metal additive precipitating from the discharge space and being in galvanic contact with the electrodes and thereby with the electric current lead-ins forming an electric integrity with the electrodes. (The metal additive generally consists of sodium amalgam in the case of the high-pressure sodium vapor gas discharge light sources.) The metal additive is in most discharge lamps present at one of the electrodes, increasing its surface area and causing thereby prolongation of the glow discharge period, i.e. the ignition phase. The electrode free of conductive metal additive or contacting only a small amount thereof is capable of transiting to the normal discharge phase in shorter time then the other electrode contacting a greater amount of the metal additive, generally increasing the surface of the related electrode. As a consequence, during the ignition process the load of the electrode contacting the metal additive, e.g. the sodium amalgam is asymmetric with respect to the load of the other electrode.
There are known gas discharge lamps comprising a starting or ignition electrode adjacent to the main electrode. In this arrangements the metal additive precipitating from the gas discharge space in the region between the two electrodes can prevent the desired influence of the starting electrode. The ignition process can be facilitated only when the main electrode and the starting electrode are electrically isolated one from another.
After longer or shorter time of operation of the gas discharge light source, i.e. the gas discharge lamp the surfaces of the sealing material and the closing ceramic element may become electrically conductive. Therefore it is possible that the basic point of the electric discharge arc in the discharge lamp is created not on the peak of the electrode but on the conductive surface either of the sealing material, e.g. sealing varnish or of the closing ceramic element, e.g. the ceramic plug. A process of this kind results in very quick deterioration of the ceramic discharge vessel.
There are known gas discharge light sources comprising special dividing means for preventing the contact between the electrodes and the conductive metal additive, e.g. the sodium amalgam.
The application of such auxiliary means causes further problems. One of them lies in the sophisticated construction of the light source and the production technology difficulties arising thereby, or, if the construction can be simplified, in the sophisticated thermal conditions of the light source: either a heat reflecting element, e.g. a ring made of niobium or tantalum should be applied for ensuring the required increased temperature of the metal additive, e.g. sodium amalgam because of the too big distance between the electrode and the metal additive, or it is practically impossible to ensure the required process of precipitating the conductive metal additive in a predetermined region of the gas discharge light source, i.e. to fulfill the requirement that in the light source be a stable cold point, the so called cold chamber.
From the GB-A 1 465 212 a high pressure sodium vapor gas discharge lamp has become known, wherein the element closing the discharge vessel is provided with a ring shaped slot for receiving the sodium amalgam. The specification doesn't disclose the dimensions of this slot and because of applying a further recess around a niobium pipe serving as a current lead-in it can not be fully ensured that the slot is the coldest place in the gas discharge light source. This is a consequence of the fact that the temperature of the recess is also relatively low, because of the heat conducting process caused by the thermally conductive current lead-in. The proposed slot is therefore not sufficient for reliable prevention of the electric contact between the sodium containing amalgam and the electrode which forms an electric integrity with the current lead-in.
The Hungarian patent specification HU-B 181 872 shows a ceramic plug element for closing the gas discharge vessel, wherein a sack like recess is made in the central part of the plug. The object of this solution is to realize the cold chamber in the plug element (and not between the plug element and the wall of the discharge vessel) and to shield thereby the metal additive from the discharge space as effectively as possible.
The German patent document DE-Al 2 405 335 discloses a high-pressure gas discharge lamp, wherein the metal additive consisting of a sodium amalgam containing substance, which is electrically conductive, is shielded from the electrode by means of a special ceramic closing element, in order to prevent flickering during operation. This solution results in very sophisticated light source construction. The patent document EP 74 188 discloses that there is really no need of shielding the electrode fully from the conductive metal additive. This EP patent document includes the proposal of completing the niobium pipe supporting the electrodes with a shoulder made of ceramic material. The height level of arranging the shoulder follows from the fact that it may not shield the sodium containing amalgam from the electrode and its smallest width is determined on the basis of the requirement that heat capacity should be enough high to prevent the process of precipitation of the amalgam on its surface. The width is exemplified to be in the range from 0.2 to 0.5 mm, and the height equal to 1.5 mm. The disadvantage of this solution is that the operation of the gas discharge lamp depends on the height of the shoulder: if the shoulder is applied higher then a heat reflecting element should be applied for ensuring the required temperature of the amalgam, and if it is lower, then after longer operation of the lamp it can not prevent the conduction between the amalgam and the electrode or the element supporting the last.
Similarly, some disadvantages are characteristic also for the solution disclosed in the European patent specification EP 188 129. The element closing the discharge tube is constructed in such way that a ring shaped channel is created between the closing element and the wall of the discharge vessel. The maximal depth of the channel is allowed to be as big as the inner diameter of the discharge vessel. In this construction the main disadvantage follows from the fact that the ceramic material of the closing element can become conductive during the operation of the gas discharge lamp and this may result in a short circuiting process between the electrode and the amalgam, especially when preparing the channel with depth as proposed by the specification.